Update Add: ?hd=1
to the end of the YouTube video URL and if it is available, an HD version will play.
Add:&fmt=18
to the end of the YouTube video URL will play the video in HQ (high quality).
Add:&fmt=22
to the end of the YouTube video URL will play the video in HD (high definition).
Regular: http://www.youtube.com/watch?v=SVvx13dpTEA
High Quality: http://www.youtube.com/watch?v=SVvx13dpTEA&fmt=18
High Definition: http://www.youtube.com/watch?v=SVvx13dpTEA&fmt=22
If a video is not available in HQ or HD, then they will not be able to be shown in those formats.